.TH std::ranges::begin 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::begin \- std::ranges::begin

.SH Synopsis
   Defined in header <ranges>
   Defined in header <iterator>
   inline namespace /* unspecified */ {

       inline constexpr /* unspecified */ begin = /*       \fI(since C++20)\fP
   unspecified */;                                         (customization point object)

   }
   Call signature
   template< class T >

       requires /* see below */                            \fI(since C++20)\fP

   constexpr std::input_or_output_iterator auto begin(
   T&& t );

   Returns an iterator to the first element of the argument.

   range-begin-end.svg

   If the argument is an lvalue or ranges::enable_borrowed_range<std::remove_cv_t<T>>
   is true, then a call to ranges::begin is expression-equivalent to:

    1. t + 0 if t has an array type.
          * If std::remove_all_extents_t<std::remove_reference_t<T>> is incomplete,
            then the call to ranges::begin is ill-formed, no diagnostic required.
    2. Otherwise,
       decay-copy(t.begin())
       (until C++23)
       auto(t.begin())
       (since C++23), if that expression is valid and its type models
       std::input_or_output_iterator.
    3. Otherwise,
       decay-copy(begin(t))
       (until C++23)
       auto(begin(t))
       (since C++23), if T is a class or enumeration type, that expression is valid and
       its type models std::input_or_output_iterator, where the meaning of begin is
       established as if by performing argument-dependent lookup only.

   In all other cases, a call to ranges::begin is ill-formed, which can result in
   substitution failure when the call appears in the immediate context of a template
   instantiation.

.SH Notes

   If the argument is an rvalue (i.e. T is an object type) and
   ranges::enable_borrowed_range<std::remove_cv_t<T>> is false, the call to
   ranges::begin is ill-formed, which also results in substitution failure.

   The return type models std::input_or_output_iterator in all cases.

   The C++20 standard requires that if the underlying begin function call returns a
   prvalue, the return value is move-constructed from the materialized temporary
   object. All implementations directly return the prvalue instead. The requirement is
   corrected by the post-C++20 proposal P0849R8 to match the implementations.

.SH Example


// Run this code

 #include <cassert>
 #include <ranges>
 #include <vector>

 int main()
 {
     std::vector v{3, 1, 4};
     auto vi = std::ranges::begin(v);
     auto vci = std::ranges::cbegin(v);
     assert(*vi == 3 and *vi == *vci);
     ++vi;
     ++vci; // OK: vci is modifiable object
     *vi = 42; // OK: vi points to mutable element
     // *vci = 13; // Error: vci points to immutable element

     int a[]{-5, 10, 15};
     auto ai = std::ranges::begin(a); // works with C-arrays as well
     assert(*ai == -5);
     *ai = 42; // OK
 }

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to           Behavior as published               Correct behavior
   P2602R2 C++20      there's machinery to prohibit certain      removed such machinery
                      non-member begin found by ADL

.SH See also

   ranges::cbegin returns an iterator to the beginning of a read-only range
   (C++20)        (customization point object)
   begin
   cbegin         returns an iterator to the beginning of a container or array
   \fI(C++11)\fP        \fI(function template)\fP
   \fI(C++14)\fP
